The xylene isomers orthoxylene, metaxylene, and paraxylene, are important chemical intermediates. Orthoxylene may be oxidized to make phthalic anhydride, which is used to make phthalate-based plasticizers among other things. Metaxylene may be oxidized to make isophthalic acid, which is used in unsaturated polyester resins. Paraxylene may be oxidized to make terephthalic acid, which in turn is used to make polymers such as polytrimethyleneterephthalate, polybutyleneterephthalate (PBT), and polyethyleneterephthalate (PET). PET is one of the largest volume polymers in the world and is used to make PET plastics (e.g., two liter PET bottles). It is also used to make polyester fiber, which in turn is used to make clothes and other fabrics. Given the large market for PET plastics and fibers, there is a substantial demand for high purity paraxylene, which is several times larger than the demand for orthoxylene and metaxylene. To help meet such demand, orthoxylene and metaxylene may be isomerized to paraxylene via use of an isomerization catalyst.
Paraxylene may be produced by reforming or aromatizing a wide boiling range naphtha in a reformer, for example, a Continuous Catalytic Reformer (CCR) or semi-regenerative reformer, followed by distillation of the naphtha reformer effluent into a C8 aromatics fraction (containing aromatics having eight carbon atoms). This C8 aromatics fraction comprises near equilibrium amounts of orthoxylene, metaxylene, and paraxylene along with ethylbenzene. The paraxylene is separated from the other components in this C8 aromatics fraction in a separation unit either by a crystallization process or by an adsorption process, thereby forming a paraxylene-depleted stream. The paraxylene-depleted stream may be further processed by passing it over a xylene isomerization catalyst in a xylene isomerization unit, wherein orthoxylene and metaxylene are isomerized to paraxylene.
Xylene isomerization catalysts may comprise a ZSM-5 zeolite support. However, these catalysts have some unwanted side reactions that consume the xylene isomers and reduce the overall xylene selectivity. Such side reactions are particularly a problem when the catalyst is of the HZSM-5 type, wherein H refers to the ZSM-5 being predominately in the hydrogen form. The HZSM-5 catalyst has several acid sites that promote unwanted cracking reactions, resulting in a relatively high amount of xylene loss and thus a decrease in the production of paraxylene. A need therefore exists to reduce the xylene losses that occur during the xylene isomerization process such that the overall xylene selectivity and thus the production of paraxylene is increased.